The team began by screening 210 non-duplicate clinical specimens, from which they recovered 120 Staphylococcus aureus isolates. Identification relied on classical microbiology, including Gram staining, catalase and coagulase testing, and mannitol fermentation, followed by molecular confirmation through PCR amplification of the 16S rRNA gene and enterotoxin genes. Methicillin resistance was determined by Kirby-Bauer disk diffusion according to Clinical and Laboratory Standards Institute guidelines, with an oxacillin inhibition zone smaller than 10 millimeters indicating resistance, and was confirmed genetically by amplification of the mecA gene. Seventeen isolates, or 14.16 percent, proved to be MRSA, while the remaining 103 were methicillin-sensitive.

To find viruses capable of killing these resistant bacteria, the researchers turned to the environment. They sampled pond water from sites around Varanasi, river water from the Ganga and Yamuna rivers and the Sangam confluence at Prayagraj, and sewage from both a municipal treatment plant and the drainage of a university hospital. Using the double agar overlay plaque assay, they isolated 47 bacteriophages, designated φRAGS1 through φRAGS47. Hospital sewage was by far the richest source, yielding 39 of the isolates, a reflection of the dense bacterial populations that phages and their hosts exchange in such environments.

When the phages were typed against the clinical isolates, six of them, φRAGS1, φRAGS8, φRAGS15, φRAGS21, φRAGS37 and φRAGS47, showed broad host-range lytic activity against MRSA strains at titers of 10¹² plaque-forming units per milliliter. Dendrogram analysis of the sensitivity profiles sorted the full collection into sixteen cluster groups. Transmission electron microscopy then revealed the architecture of the six lead phages: two, φRAGS8 and φRAGS37, carry an icosahedral head with a short, non-contractile tail, placing them in the Podoviridae-like morphotype, while four, φRAGS1, φRAGS15, φRAGS21 and φRAGS47, possess long contractile tails characteristic of the Myoviridae-like group, classified according to International Committee on Taxonomy of Viruses criteria.

Molecular characterization reinforced the morphological split. Restriction digestion of purified phage DNA with the hexa-cutter enzymes EcoRI and HindIII produced distinct fragment patterns consistent with two separate viral families, and low-melting-temperature ERIC-PCR fingerprinting, performed after three sequential plaque passages, confirmed that the genomes remained stable across propagation cycles. Notably, a polar plot of lytic activity showed that φRAGS15 lysed all 17 clinical MRSA strains, marking it as the strongest single candidate for future development, while φRAGS21 showed comparatively weaker activity across the panel.

The second pillar of the strategy is the implant material itself. The researchers worked with carbonate-intercalated Mg²⁺/Al³⁺ layered double hydroxide nanoparticles, known as nanoLDH, incorporated at 1 percent by weight into poly(methyl methacrylate) bone cement, the standard acrylic grouting material used in hip and knee replacements since its FDA approval in 1969. The resulting nanocomposite was characterized by X-ray diffraction, scanning and transmission electron microscopy, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis. Previous work by the group had already shown through hemolysis, thrombogenicity, simulated body fluid apatite formation and osteoblast cell culture assays that the nanohybrid cement is more biocompatible than conventional PMMA and supports bone-forming activity.

To test the therapy, 24 female rabbits received prostheses implanted into a drilled defect in the distal femoral metaphysis under ketamine and midazolam anesthesia. The animals were divided into a control group, a group receiving conventional bone cement, and a nanoLDH group that was further split into an implant-only subgroup and an implant-plus-phage subgroup. Six days after surgery, an MRSA inoculum of 5 × 10⁶ colony-forming units was injected intramedullary to establish infection, which was verified by culture, susceptibility testing and mecA amplification. Once infection was confirmed, rabbits in the phage group received subcutaneous injections of a lyophilized six-phage master mix, at 10¹² PFU/mL and a dose of 15 microliters per kilogram, into the soft tissue surrounding the implant on days 12, 15 and 18.

The outcome data showed a clear therapeutic gradient across the groups. Untreated infected rabbits and those with conventional cement developed progressive cortical erosion, and the cement group additionally showed osteolysis, reactive sclerosis and sequestrum formation, the hallmarks of chronic implant-associated osteomyelitis. Rabbits with nanoLDH alone fared better, with reduced osteolysis and early new bone formation, but the most dramatic improvement appeared in the phage-treated nanoLDH group: radiographs on day 18 revealed substantial restoration of cortical architecture, marked new bone around the implant and no progression of sequestra, while cultures from the implant site turned negative after treatment. Histopathology mirrored these findings, with near-complete disappearance of inflammatory infiltrates, minimal necrosis, woven bone bridging cortical defects and restored marrow architecture in the phage-treated animals, compared with dense neutrophilic infiltration, abscesses and extensive necrosis in the cement group.

Recovery timelines underscored the magnitude of the effect. Animals receiving the nanoLDH-phage combination recovered in about 18 days, roughly half the 36-day recovery observed in untreated controls, with nanoLDH alone at 24 days and conventional cement at 30 days. Statistical analysis using the non-parametric Kruskal-Wallis test followed by Dunn’s multiple comparison test with Bonferroni correction confirmed significant differences among all groups. The authors are careful to note the limits of their claims: they did not directly demonstrate sustained phage release from the cement or definitively prove a mechanistic synergy between the nanomaterial and the viruses, and the phages were delivered by subcutaneous injection at the implant site rather than embedded in the prosthesis itself.

Even with those caveats, the study adds a technically detailed data point to the growing field of phage therapy for orthopedic infections, where staphylococcal species account for roughly four out of five prosthetic joint infections and two-thirds of cases involve S. aureus. Because phages recognize specific bacterial surface receptors and replicate at the site of infection, they offer strain-specific killing that antibiotics such as vancomycin, linezolid and daptomycin, each increasingly hampered by resistance and side effects, cannot match. The researchers suggest that future work should compare phage-loaded prostheses directly against conventional antibiotic therapy and standalone phage treatment, and pursue whole-genome sequencing to confirm the absence of undesirable genes in the φRAGS isolates. If those steps succeed, locally delivered phage cocktails paired with osteoconductive nanohybrid cements could become a credible alternative to antibiotic-loaded bone cement for managing multidrug-resistant implant infections.

Subject of Research: Bacteriophage therapy combined with nanohybrid bone cement for treating MRSA prosthetic joint infections

Article Title: Synergistic application of φRAGS bacteriophages and nanohybrid bone cement-based joint prostheses against methicillin-resistant Staphylococcus aureus

Article References: Mishra, R. R., Mishra, P., Kishor, C., Kumar, A., Kumar, A., & Nath, G. (2026). Synergistic application of φRAGS bacteriophages and nanohybrid bone cement-based joint prostheses against methicillin-resistant Staphylococcus aureus. Frontiers in Virology, 6, Article 1880302. https://doi.org/10.3389/fviro.2026.1880302

Image Credits: AI Generated

DOI: 10.3389/fviro.2026.1880302

Keywords: bacteriophage therapy, MRSA, prosthetic joint infection, bone cement, nanoLDH, Staphylococcus aureus, antimicrobial resistance, PMMA, rabbit model, osteomyelitis, φRAGS phages, biomaterials